The invention relates to a method for adjusting current levels, which have been falsified by offset-current levels, in a subscriber line circuit located inside a digital telephone exchange.
Such an adjustment method is known in the form of an offset current adjustment method. The current flow on the wires of a subscriber line leading to a subscriber line circuit is interrupted by at least one relay in each case. After such an interruption, the offset-current level of the subscriber line circuit is determined through the use of a processor controlling that subscriber line circuit, in such a way that the offset current is measured a number of times. A mean value is formed from the measured offset-current levels, and the result is stored in a memory associated with the processor. If a current measurement is carried out during normal operation, for example for testing the operability of the subscriber line circuit or for adjusting the supply current for a subscriber line, the current measurement is carried out in approximately the same way, and the result is in this case corrected by the offset-current level.
The above described method is disadvantageous, because the method for current measurement carried out during normal operation as well as the adjustment of the previously established offset-current level are integrated in the firmware of the processor which controls the subscriber line circuit. Since the current levels are usually to be determined precisely in a range of microamperes (xcexcA), the sampling rate for such current measurements should be as high as possible. The firmware for the processor for controlling the subscriber line circuit is, however, configured such that the sampling rate of, for example, 1 kHz cannot be increased any further when using this method.
A further disadvantage of this method is that any disturbance variable present in the subscriber line circuit, such as the offset current in a current measurement, is eliminated only on the basis of the firmware, but still exists in the subscriber line circuit.
This means that, for example, an adjustment of the supply current in the low-voltage section of the subscriber line circuit, whose value is normally in the range of approximately 25 mA with respect to the wires of the subscriber line, is made very difficult insofar as the offset-current level present in the low-voltage section falsifies the supply current level by about 1 mA.
A current-voltage adjustment method which relates to a subscriber line circuit is known from International Publication No. WO-A-96/27970. The subject matter disclosed in this publication describes a DC control circuit, which includes a low-pass filter to identify DC voltage fluctuations, a converter to evaluate an incremental signal, a threshold value identification element to activate compensation functions, and an accumulator, in order to produce a DC compensation voltage by incremental steps. This DC compensation voltage is converted into an analog compensation signal, and this compensation signal is then subtracted from an input voltage, which is intended for an amplifier within a so-called SLIC (Subscriber Line Interface Circuit), in order to compensate for any DC voltage fluctuations. This procedure allows the voltage fluctuations arriving at the SLIC to be compensated for. Since the conditions for incoming voltage fluctuations are different from those for offset currents that are present in the subscriber line circuit, the above described conventional compensation method is regarded as being unsuitable for the adjustment of current levels falsified by offset currents.
It is accordingly an object of the invention to provide a method for adjusting current levels falsified by offset-current levels which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for adjusting current levels falsified by offset-current levels, which includes the steps of:
providing a subscriber line circuit in a digital telephone exchange, the subscriber line circuit including a high-voltage section and a low-voltage section, the high-voltage section mainly including line drivers and elements for a subscriber line indication, the low voltage section including a signal processor and serving essentially as an analog/digital interface and providing a 2-wire/4-wire conversion;
measuring, in the low-voltage section, a level of an offset current when the offset current is present, the level of the offset current having a given mathematical sign;
storing, in the low-voltage section, the level of the offset current with a mathematical sign opposite to the given mathematical sign for providing a stored level of the offset current; and
adding, in the .low-voltage section, the stored level of the offset current with the mathematical sign opposite to the given mathematical sign to a current to be measured for a correction when a current measurement is to be carried out during a normal operation.
In other words, the invention solves the object of the invention in particular in that both the current measurement method and the adjustment method are integrated in the low-voltage section of the subscriber line circuit. A previously measured offset-current level for electronic components associated with the subscriber line circuit is in this case stored with the opposite i.e. reversed mathematical sign in the low-voltage section of the subscriber line circuit, and when a current measurement is to be carried out during normal operation in the low-voltage section, for example for testing the subscriber line circuit or the supply current, the stored value is added to the current to be measured.
The current measurement thus includes only current levels which have already been corrected for the offset-current levels. This is advantageous since, firstly, there is no need to apply the correction for the offset-current level after the current measurement and, secondly, the current level, for example that of a supply current, can be adjusted considerably more easily without the influence of the offset current. Apart from this, the method according to the invention. achieves a further advantage in that a higher sampling rate can be achieved for a current measurement. This is done by no longer including the current measurement method in the firmware of the processor which controls the subscriber line circuit, but by including it in the hardware of the low-voltage section.
In accordance with an advantageous mode of the invention, a current is measured for the purpose of testing the subscriber line circuit or for adjusting the supply current.
In accordance with another mode of the invention, the measuring step includes measuring offset-current levels of components in the subscriber line circuit.
Furthermore, the storage of the offset-current level in the low-voltage section is advantageously configured such that the offset-current level can be adjusted from the outside or externally. With an appropriate adaptation of the offset-current level, it is possible in some cases to avoid a subsequent adjustment of the current measurement method.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for adjusting current levels, which have been falsified by offset-current levels, in a subscriber line circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.